Photosensitive detector systems, commonly known as light curtains, are employed in a variety of industrial applications to sense the intrusion of objects in or around a prescribed area. Light curtains typically are employed for operator protection around machinery such as punch presses, brakes, molding machines, presses, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like. Conventional light curtain systems employ invisible pulsed infrared light beams which project across the area to be protected. Unintended intrusion of the light beams by an object, such as the operator's hand, are sensed by the circuit to trigger a warning signal, shut the machinery down, or otherwise safeguard the area.
There is a critical requirement to provide a light curtain system which cannot fail unblocked, i.e. in an unsafe mode. Thus, certain governmental regulations concerning industry workers prohibit the use of machinery having a design in which a part can fail unsafe. Conventional light curtain systems have design limitations which can permit them to fail unblocked, making them unsafe for certain applications.
Another problem with conventional light curtain systems is their high degree of complexity and cost. Certain of these systems, such as the Weber U.S. Pat. No. 4,266,124, provide a system which attempts to achieve self-checking operation by using one set of circuits to select individual light receivers and a separate set of additional circuits to verify that the correct receivers are selected. This results in a relatively more complex and costly design.
Conventional light curtain designs employ a light receiver circuit with a series of phototransistors which respond to light signals. Each phototransistor typically is coupled with a single operational amplifier to provide a fast and sensitive circuit, but which on the other hand is relatively complex, expensive and failure prone.
Conventional light curtain systems also typically employ analog circuits to sequentially select the photodetector channels. These circuits produce a relatively unsafe system in that intruding objects may not be properly detected if an incorrect channel is selected such as from a part failure. The system thus would not be intrinsically safe.